U.S. Pat. No. 4,846,191, of Brian P. Brockway et al. (hereinafter the Brockway et al. '191 patent) discloses a pressure measurement device for sensing physiological pressures. The device consists generally of a pressure transducer with associated electronics and a pressure transmission catheter. The distal tip of the catheter senses the pressure of the target site and transmits the pressure fluidically through the catheter to be sensed by the pressure transducer.
In one embodiment of the Brockway et al. '191 patent, the pressure transmission catheter consists of a small diameter hollow tube which is filled with a low viscosity liquid. This liquid is in fluid communication with the pressure transducer at the proximal end of the hollow tube and a gel-like material at the distal end. The gel-like material provides a direct interface with the tissue or body fluid of which the pressure is to be measured.
Pressure measurement catheters such as described in the Brockway et al. '191 patent have been used to augment/support therapeutic treatments, as well as provide valuable information for diagnosis. For example, blood pressure measurements are very important when percutaneous therapeutic catheters affect blood pressure. Examples of therapeutic catheters include intra-aortic balloon catheters, angioplasty catheters, and perfusion catheters.
Taking one type of therapeutic catheter as an example, intra-aortic balloon catheters are designed to assist a failing heart through cyclic inflation-deflation of a balloon placed in the descending thoracic aorta, in counterpulsation to the heart. In a typical procedure, a guidewire (a thin flexible wire) is inserted through an incision into the common femoral artery and is directed through the delicate, tortuous and narrow vasculature. Once the guidewire is positioned, the intra-aortic balloon catheter is passed over the guidewire, utilizing the guidewire lumen of the catheter, until the balloon reaches the desired location.
The balloon is connected through a series of thin tubes to a control system which controls the balloon's inflation and deflation, repeatedly, in synchrony with a patient's heart beat. The action of the balloon assumes some of the load of the heart mainly by increasing systolic pressure which increases the flow of blood through the coronary arteries. In order to synchronize the balloon inflation/deflation with the heart beat, the patient's heart electric signal or electrocardiogram is detected using surface electrodes attached to the skin of the patient. These electrodes are connected to the control system of the intra-aortic balloon catheter. Inflation of the balloon occurs at a specified time relative to a reference signal on the patient's electrocardiogram.
The intra-aortic balloon catheter system of the Brockway et al. '191 patent, as described above, has its limitations. For example, the surface electrodes used to detect the patient's electrocardiogram for balloon inflation/deflation control have a limitation in that the signal can be relatively weak and noisy, leading potentially to spurious or unreliable responses. Further, the electrodes may become disengaged from the patient's skin due to lack of adhesion or being knocked off. Additionally, the patient typically has one set of surface electrodes attached for general monitoring; adding a second set of electrodes for controlling the intra-aortic balloon catheter adds further complexity and discomfort for the patient.
Blood pressure is typically used to calibrate the intra-aortic balloon catheter system. Ideally, this pressure should be measured in the vicinity of the catheter balloon. The electric signals sensed by the electrodes are used as the primary trigger for the catheter control system to pneumatically inflate the balloon, and the pressure signals are used to temporally calibrate balloon inflation to the electrical signals.
In the treatment modality of the Brockway et al. '191 patent, once the therapeutic catheter is placed, the guidewire is removed from the catheter's guidewire lumen. The guidewire lumen is then flushed with saline, or saline with anticoagulation agents such as heparin, in order to "fill" the guidewire lumen with a liquid. Once filled, the proximal end of the catheter is connected to a pressure transducer. In this respect, blood pressure upstream of the balloon is fluidically transmitted through the saline and detected by the pressure transducer.
This approach has its limitations for accurately measuring the pressure, especially with smaller balloon catheters having small guidewire lumens. Limitations may include high system compliance, the presence of air bubbles in the guidewire lumen, and possible blood coagulation in the guidewire lumen. Compliance is a property of this measurement system that provides a measure of resistance to deformation due to pressure. A system with high compliance will deform more than a low compliance system as pressure is increased. A high compliance system will tend to absorb rapid pressure changes that should be transmitted through the liquid in the lumen. This, in tun, reduces the accuracy of the pressure measurements as a result of lowering the frequency response. Excessive air bubbles and thrombus in the guidewire lumen can also result in dampening and loss of accuracy of the measured blood pressure signal. This can reduce the efficacy of the inflation of the balloon.
A further limitation of the system of the Brockway et al. '191 patent is that it is labor intensive. There is the additional preparation required by the user to fill the guidewire lumen just prior to use. Improper filling may lead to bubble formation in the catheter lumen. It is also necessary to flush the lumen to remove thrombus that may form, otherwise the pressure signal might be blocked altogether.
What is needed is an improved apparatus to obtain more reliable and higher quality measurements of blood pressure. An improved apparatus for sensing an electric signal is also needed.